The present invention resides in a leak proof nozzle assembly in a hot runner injection molding apparatus.
Hot runner nozzle tips may include single or multiple outlet channels for feeding molten resin to a mold cavity in an injection molding system. The hot runner nozzle tips are generally spaced from the injection gate leading to the mold cavity. The spacing is necessary to avoid direct contact between the cooled mold cavity by the heated nozzle tip in order to prevent the nozzle tip from rapidly cooling. Rapid cooling of the nozzle would cause the resin to freeze up and render the nozzle inoperable. In addition, if the nozzle tip is heated from a cold condition to its operating temperature, generally 350.degree.-450.degree. F., space must be provided for thermal expansion which results in the nozzle and the nozzle tip growing in length.
Conventional hot tip designs usually provide a bubble or resin space surrounding the tip and filled with resin. The resin in the resin space has poor thermal conductivity compared to steel and effectively insulates the hot tip from the cooled mold cavity. U.S. Pat. No. 4,312,630 is an example of this design and shows the heated tip insulated from the mold cavity by a bubble well or resin space. Since the resin in the bubble well is subjected to high injection pressure (around 20,000 psi) it must be effectively sealed. This is accomplished in U.S. Pat. No. 4,312,630 by the close fit between the nozzle shell and the sides of the well.
This method of sealing is not entirely satisfactory and was further improved by making the sealing flange of the nozzle from a separate material, generally titanium, so that less heat would be conducted from the heated nozzle body to the mold cavity. Examples of this are shown in U.S. Pat. Nos. 4,053,271, 4,557,685, 4,768,945 and 4,771,534. The sealing action is achieved while permitting the longitudinal thermal expansion of the nozzle to occur unrestricted. The sealing rings will slide within the cylinder bores of the bubbles and wilI deform inwardly to maintain the seal. Furthermore, the internal injection pressure of the resin acting on the nozzle surface of the seal tends to urge the skirt of the seal against the sides of the bubble walls to enhance the sealing action.
It was soon realized, however, that the comparatively large volume of resin contained in the bubble or resin space tended to remain there indefinitely. Some resins could tolerate this extended dwelling time at their processing temperatures, but others could not. The resins which could not tolerate the extended elevated temperature dwell are known as heat sensitive resins and soon degrade when exposed to extended periods of time at their normal processing temperatures. Typical of these heat sensitive resins are polyacetal, polyvinyl chloride and polyethylene terephthalate.
In order to solve this problem, sealing arrangements were developed to minimize and in some cases entirely eliminate the bubble or resin space while at the same time attempting to maintain good thermal insulation between the nozzle tip and the mold cavity and also to provide space for thermal expansion of the nozzle tip. Examples of these designs are shown in U.S. Pat. Nos. 4,043,740, 4,286,941, 4,344,750 and 4,981,431. In these cases, small titanium seals are used to locally contain the melt channel between the nozzle and the mold cavity gate. However, the disadvantage of these designs is that they all employ titanium alloy seals. Although titanium has almost a quarter of the thermal conductivity of steel, it does not compare as well to the insulating properties of resin or air, the other materials used for insulation in nozzle construction. Therefore, while solving the resin degradation problem, these designs required more heat energy to be provided in order to render them effectively operable. Furthermore, the small seals were usually required to deflect in order to insure an effective seal. Due to imperfect manufacturing methods, the required sizes of the seals, bubble dimensions and other variables meant that some seals were deformed beyond their elastic limit and failed while others did not make contact at all. In both cases, resin leakage would occur.
U.S. Pat. Nos. 4,662,837 and 4,682,945 show another approach to the sealing problem. In these cases, a high temperature plastic seal is used to effect the seal between the nozzle and the mold cavity. Typically, plastic has thirty times less thermal conductivity than titanium alloys. However, the high temperature seal may be subject to cracking and degradation caused by high molding pressure and high processing temperatures. Further approaches are shown in U.S. Pat. Nos. 4,161,386, 4,266,723 and 5,028,227.
Accordingly, it is a principal object of the present invention to develop an effective hot runner nozzle assembly using a heated injection nozzle and a cooled mold cavity communicating therewith. It is a further object of the present invention to develop an improved hot runner nozzle assembly as aforesaid which is leak proof, conveniently prepared and easy to use in practice.
It is a still further object of the present invention to develop an improved hot runner nozzle assembly as aforesaid which enables expansion of the nozzle, uses a resin space or bubble adjacent the nozzle tip and is not subject to resin degradation in the resin space.
Further objects and advantages of the present invention will appear hereinbelow.